Skip Windows-hanging shm IPC test

`test_parent_writer_child_reader` deadlocks on Windows (the
parent/child shared-mem transfer hangs at the larger frame size),
so the `windows-latest` CI leg ran to the 16-min job cap instead
of completing. It's a genuine nascent-Windows shm bug, not a
clean "unsupported", so it's `skipif`'d (not removed) and tracked
under #404; `test_child_attaches_alot` still runs on Windows.

- `@pytest.mark.skipif(platform.system() == 'Windows', ...)` on
  the parametrized `test_parent_writer_child_reader` so the leg
  completes + reports. linux/macOS unaffected (all 6 variants
  still run).

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tests/test_shm.py

@@ -120,6 +120,13 @@ async def child_read_shm_list(
                 print(f'(child): reading frame: {frame}')
 
 
+@pytest.mark.skipif(
+    platform.system() == 'Windows',
+    reason=(
+        'parent/child shm IPC deadlocks on Windows '
+        '(frame-size dependent hang); nascent — see #404'
+    ),
+)
 @pytest.mark.parametrize(
     'use_str',
     [False, True],